The present invention relates to pruning tools and, more particularly, to hand-held pruning tools which are hydraulically operated.
The present invention relates more generally to hydraulically operated tools which use two strokes, a powerful stroke (hereinafter referred to as the "power stroke") requiring a relatively large force, and a relatively weak stroke, typically used to reset the device so that it is ready for the next powerful stroke (hereinafter referred to as the "reset stroke"). For ease of exposition, and without in any way limiting the scope of the present invention, the descriptions and illustrations included herein are confined to a hydraulic pruning tool.
Pruning tools are widely used in agriculture and forestry, as well as in private gardens, to trim and shape trees, shrubs, and the like.
A typical pruning tool includes a cutting head, which typically includes a scissors-like mechanism made up of two pivotally connected blade members, and a handle which allows the operator to bring the cutting head to the appropriate location and to activate the cutting head.
The scissors-like portion of the cutting head of a typical pruning tool is normally in the open position, with the distance between the two blades defining the size of the largest branch or limb which can be cut by the tool. In operation, the cutting head is maneuvered so that a limb is admitted between the blades, at which point the operator activates the cutting head, forcing together the two blade members so as to cut a branch or limb. The amount of force needed to cut a branch depends on a number of factors, including, but not limited to, the size and nature of the limb being cut.
Once a limb has been cut, additional force, typically of much lower magnitude than the cutting force, must be applied to the cutting head in order to separate the blades and prepare the cutting head to receive the next limb to be cut.
Various means may be used to power the cutting action, or power stroke, of the cutting head. These can range from manual force supplied by the fingers and/or hand of the operator, to an electrical motor, to a suitable internal combustion engine or to pneumatic or hydraulic mechanisms, among others.
The same or related means may be used to supply the relatively small force needed to reopen the cutting head blades following the cutting action during the reset stroke. Such a force may alternatively be supplied by various spring, or other, biasing mechanisms.
In various heavy-duty pruning applications, hydraulic systems have become the power source of choice. Hydraulically operated pruning tools are typically preferred over pneumatic tools since the pressures generated by hydraulic tools can be 10 to 15 times larger than those achieved with compressed air. This allows a hydraulic tool to be more compact, lighter and more powerful than its pneumatic counterpart. Furthermore, a hydraulic tool using oil needs less maintenance since it is self-lubricating.
A variety of hydraulic pruning tools are available. Typically, the reset stroke of such tools is powered by one of two basic means. In one type of device, a double acting hydraulic piston is used to actively alternately drive the power stroke and the reset stroke. Such an arrangement is relatively complicated and expensive to build and maintain. Furthermore, such as system is inefficient, since energy must be actively supplied for the resetting of the tool.
In another type of device, a spring mechanism or similar biasing mechanism, is provided to force the blades apart once the power stroke is over. Such mechanisms involve certain inefficiencies in that during the power stroke additional energy, beyond that required for the actual cutting of the limb, has to be expended to overcome the biasing force of the spring. Furthermore, because of the back pressure in the system, a spring mechanism is typically slow to operate, which could reduce the overall efficiency of the pruning operations. Finally, spring mechanisms may suffer from non-uniformities in their elasticities over the length of the spring and/or with time, which could require significant maintenance.
There is thus a widely recognized need for, and it would be highly advantageous to have, a relatively uncomplicated and low maintenance hydraulic pruning tool, or similar device, which would quickly and efficiently reset, or open, the cutting head of the tool without relying on double-acting pistons or spring-like mechanisms.